The sense of smell permits animals to find food, recognize mates, and avoid predators. It is possible to study this complex sensory modality at the level of olfactory genes, neurons, and circuits in the powerful genetic model organism, the Drosophila fruit fly. In both mammals and insects, olfactory neurons have evolved to detect odors associated with food. Complex neural circuitry must be brought to bear upon the problem of discriminating attractants in high quality food from repellents that might signify food tainted by spoilage. Drosophila has two olfactory sensory organs, the antenna and maxillary palp, but the relative role of these two structures in general food odor detection is poorly understood. The proposed experiments will address the possible role of the maxillary palp in detecting specific repellents. Genetic mapping experiments will be performed to elucidate the wiring logic of the maxillary palp as it innervates the first olfactory relay in the brain. Using a behavioral genetic approach, we will define the role of identified maxillary palp neurons in detecting and mediating avoidance of certain repellent stimuli. The results will increase understanding of the neural circuitry that underlies stereotyped feeding behaviors important in all species.